Electronic storage tubes employing targets of the "coplanar grid" type are already in use and have a capability of storing an image and retaining the stored image even after repeated write operations. Electronic storage tubes of the type to be described herein in greater detail have three basic operating modes, namely a read mode, a write mode and an erasure mode. The write mode is typically performed after completion of an erase mode which consists of developing a substantially uniform charge pattern upon the coplanar grid of the target which charge pattern prevents the electron beam from striking the target conductive member so as to yield a uniform "black" picture.
In the write mode, the substantially uniform charge pattern is modified by increasing the target voltage level to a value sufficient to enhance the secondary emission of the coplanar grid when struck by a modulated electron beam such that a greater member of electrons are "knocked off" of the coplanar grid than are retained thereby due to the striking of the coplanar grid by the electron beam at high velocity. The "knocked off" electrons are collected by the deceleration grid mesh of the tube. This results in a modified surface charge pattern which is more positive at those locations where a beam of greater electron density has struck the coplanar grid.
During the read mode, the target voltage is significantly reduced and an unmodulated electron beam (i.e. of constant beam current) is caused to scan the target. The surface charge pattern upon the coplanar grid serves in a manner analogous to the control grid of a vacuum tube triode selectively controlling the amount of electrons from the electron beam permitted to strike the target conducting surface as a function of the charge pattern. Since the charge pattern, although non-uniform, is more negative than the potential at the cathode of the electron gun structure, no electrons strike the coplanar grid structure enabling the image formed during the write mode to be retained and thereby permitting repeated read operations without image loss.
It has been found that electrons impacting the deceleration grid mesh during read operations generate ionizing radiation (i.e. X-radiation) which greatly increases the conductivity of the coplanar grid material causing an undesirable decrease in the retention time.